1. Field of the Invention
The present invention relates to a compound lens system. In particular, the compound lens system encompasses four lenses and is specifically used in a handy scanner of a computer system for photographing a printed object.
2. Description of Prior Art
A handy scanner is a device coupled via an interface to a computer system for photographing a printed object into a digital image. As shown in FIG. 1, the handy scanner consists essentially of a lens system 1 for photographing a printed object 3, and a charge coupled device (CCD) 2 for converting the photographed image into a digital image. The digital image can be displayed on a graphic monitor (not shown) connected to the computer and be further processed.
Currently available handy scanners are capable of providing a resolution power for the digital image in the range from 200 DPI to 400 DPI (DPI stands for dot per inch). The lens systems incorporated in these handy scanners are only capable of photographing a width of 105 mm (4.13 in) of the printed object into acceptable sharpness (this width will be hereinafter referred to as the "scanning width"). For objects located outside this width of 105 mm, the photographed image thereof is blurred enough not to be able to be converted into a digital image with acceptable resolution. The blurred edge of the photographed image is due to the aberrations of the lens system.
A usual design purpose of a scanner lens is therefore to cope with the aberration problems such that the scanner lens is capable of photographing a wider dimension of the printed object into sharp image, i.e. offers a wider scanning width.
The scanning width generally can be increase by increasing the dimensions of the lenses included in the lens system of the scanner. However, for a trend leading to small and compact peripheral devices for a computer system, a handy scanner with a large dimension is mostly unfavorable.